The invention generally relates to an ultrasonic transducer. More particularly, it relates to one for use in proximity switches.
Ultrasonic proximity switches are used in automation engineering, mainly for contactlessly sensing the presence or distance of objects. Depending on the measuring task, the bound method or the echo-delay-time method are used. In the bound method, an ultrasonic transmitter sends out signals which reach an ultrasonic receiver on a direct path. The object to be sensed interrupts the sound path and is detected in this way. In the echo-delay-time method, on the other hand, the ultrasonic echo reflected by the object to be sensed is received and the distance of the object is determined from the signal delay time between transmission and reception.
Key components for both methods are the ultrasonic transducers. In the transmitting case, they are used for converting electric signals into sound waves and, in the receiving case, they are used for converting sound waves into electric signals. In the case of devices based on the echo-delay-time method, often one and the same transducer is used alternately for transmission and reception. This reduces the expenditure on equipment, but fixes a minimum distance below which no measurements are possible because of the unavoidable decay processes of the transducer after the transmitting cycle.
Ultrasonic transducers are available in various technical forms. For industrial use, solid-state transducers are usually used, because of their robustness. They basically include a piezoceramic device as an element for converting between electric signals and acoustic signals and a resonant adapter layer, with which the transfer of sound to the air is optimized.
Typical examples of arrangements of this type are shown, inter alia, by DE 25 41 492 B2 and DE 196 30 350 C2. For use in practice, the ultrasonic transducer must be secured in a suitable way, without its function being impaired as a result. For this purpose, plastic moldings and polymer foams are usually used, see for example DE 198 09 206 A1. The polymer foams also bring about a desired mechanical insulation of the ultrasonic transducer. Furthermore, electrical shielding can be performed by metal pots additionally introduced into the arrangement.
Ultrasonic transducers of the type described are used in large numbers in industrial proximity switches and have proven successful in operation. In the course of miniaturization of the devices, however, problems are increasingly being caused by the coupling-over of structure-borne sound to the ultrasonic transducer, since the layer thickness of the enveloping polymer foam layers decreases, and consequently so does their insulating capacity with respect to an undesired radial flow of sound. The devices are consequently sensitive to disruptive noises in the region of their operating frequency, which can be mechanically coupled into the proximity switches from surrounding machine parts if fixed mounting is used. Furthermore, there is the risk of part of the transmitted sound escaping laterally into the surrounding machine parts and leading there to undefined echoes, which in turn can be coupled back to the proximity switch.
The problem described has so far not been solved satisfactorily. Improvised attempts to do so use a reduced sensitivity of the proximity switch, which however is disadvantageous for normal operation. A further attempted solution is to make the ultrasonic transducer protrude from the front of the housing sleeve, so that the transmission path between the acoustically active part of the transducer and the surrounding structural parts via which the structure-borne sound could be transferred is increased.
One particular implementation of this principle is specified in DE 38 32 947 C2, in which the adapter layer is extended in a thin-walled and tubular form over the rear side of the piezoceramic device, this extension amounting to approximately one fourth of the length of the sound path. In this region, the transducer is held via a flexible clamping ring, whereby the transmission of structure-borne sound is distinctly reduced. Disadvantages of solutions of this type are that the transducer protruding from the contour of the device is susceptible to damage and often the overall length of the proximity switches is also increased.
It is therefore an object of an embodiment of the present invention to specify a device for transmitting and receiving ultrasound. Preferably this is used for ultrasonic proximity switches. Even more prefereably, the device is one which is insensitive to transmission of structure-bome sound and/or, at the same time, one which avoids at least one of the disadvantages of the described known attempted solutions.
At least one of the objects can be achieved according to an embodiment of the invention by an ultrasonic device. Preferably, the device includes:
a) a housing in which an ultrasonic oscillator, formed by a piezoceramic device and an adapter layer, is secured with a nonpositive and/or positive fit,
b) the nonpositive and/or positive fit is achieved by at least four layers with acoustic wave impedances that vary to alternating degrees,
c) the layers are arranged in the following sequence, considered from the ultrasonic oscillator,
d) the ultrasonic oscillator is embedded in a first acoustically soft layer with at least one flexible insulating material as a component part,
e) the first layer is surrounded by a second layer, which consists of at least one acoustically hard material, preferably metal,
f) lying around the second, acoustically hard layer is a third, acoustically soft layer, which surrounds the second layer at least in the direction directed from the ultrasonic oscillator radially outward toward the housing and which comprises one or more plastics in foam form, the density of which is always less than 0.6 kg/dm3, and
g) the third layer is surrounded at least partially by a fourth layer with a high acoustic wave impedance.
Acoustically soft and acoustically hard materials are understood as meaning those materials of which the wave impedance, defined as the product of the material density and material wave velocity, is very low or very high, respectively. The succession of layers, according to an embodiment of the invention, with acoustically soft and acoustically hard materials in alternation, has the effect that the structure-borne sound coupled over from the ultrasonic oscillator outward into the housing and the flow of structure-borne sound directed back to the ultrasonic oscillator encounter a great mismatch; at the layers of differing acoustic hardness there always occurs almost total reflection in each case, so that the overall transmission is reduced to a minimum. The structure-borne sound insulation is in this case all the better the greater the differences in the wave impedances at the individual layers.
The fourth layer may represent a housing of the ultrasonic transducer. The mismatches of the wave impedance in the layers one to three are generally already so effective in the construction according to an embodiment of the invention that, for this layer, even conventional plastics with a wave impedance which is lower than that of metals are adequate to provide structure-borne sound insulation.
It is particularly advantageous if the third layer of the ultrasonic transducer includes a plastic with a density of less than 0.2 kg/dm3, since particularly good structure-borne sound decoupling is achieved for this. This increased structure-borne sound decoupling may be necessary in the case of installation conditions of the transducer that are very unfavorable in terms of structure-borne sound and/or in the case of very high signal amplification of the sensor electronics.
In the production of the transducers, it is only with some effort that layers with such low densities can be introduced as a casting compound, especially if the layer thickness in the case of small structural forms is very thin. It is therefore advantageous to use prefabricated foam moldings for the third layer.
Furthermore, it is particularly advantageous if enamel-insulated high-frequency stranded wire with a total cross section of less than 0.05 mm2 is used for the electrical connection of the ultrasonic transducer. In the case of conventional connecting leads, structure-borne sound is transmitted to a disruptive extent via the conductor or the stranded wire and/or via the insulation, which generally includes plastics, such as for example PVC, PU, Teflon or the like.